Process and apparatus for the improved combustion of liquid fuels



Nov. 22, 1960 H. M. Fox

PROCESS AND APPARATUS FOR THE IMPROVED comausnou 0F LIQUID FUELS Filedllay 27, 1955 INVENTOR. H. M. FOX

f/wmq A r TORNE vs nite States Patent Qfice Patented N 0v. 22, 1 960rnocnss AND APPARATUS Fag THE mnovnn' iIGMBUSTIGN on LIQUID FUELS HomerM. Fox, Bartlesvilie, 01th., assignor to Phillips Petroleum Qornpany, acorporation of Delaware Filed May 27, 1955, Ser. No. 511,461

11 Claims. (Cl. 60- 3936) This invention relates to jet engines Inoneaspect, this invention relates to a method for improving thecombustion of fuel in a jet engine. In a more specific aspect, thisinvention relates to an improved jet enginewhich comprises a system fortransforming fuel supplied to the jet engine into a fuel having moredesirable combustion characteristics.

The most important element of a jet engine aircraft is the power plant,and this, in turn, is only as good as its combustion system. The studyof the combustion chamber and the mechanics of combustion is of primeconcern.

The purpose of the combustion chamber is to convert the chemical energyof hydrocarbon fuels into thermal energy. This energy is absorbed by theair flowing through the engine to provide the high-velocity exhaust jetnecessary for propulsion.

The desired properties of jet engines include low frontal area forminimum drag, low weight, and operational flexibility. In some respects,frontal area and weight are inversely related to the heat release rateof the combustor employed. Thus, for a given thrust rating, a combustionprocess yielding a high heat release rate (expressed as B.t.u./hr./cu.ft.) will generally permit the use of an engine design having lowerfrontal area and weight than will a combustion process having a lowerheat release rate. In other words, higher heat release rates willgenerally permit the design of engines ha ing higher thrust per unit ofengine weight and per unit of engine frontal area. Heat release ratesdepend, in turn, on the stability and efiiciency of the combustionprocess, as well as on the heating value of the fuel It is known thatfuels having high flame speeds, e.g., hydrogen, acetylene and ethylene,possess high combustion stability and efliciency. Additionally, the fuelmust be in the gaseous or vaporous state before combustion can occur.Consequently, "a gaseous or vaporized fuel requires a shorter overallresidence time in the combustor for complete combustion than does a fuelinjected in the liquid state since the'latter must'first be vaporized.The net result is that low molecular weight fuels, such as hydrogen,acetylene and ethylene provide higher thrust per unit of engine Weightand'of frontal area than does a high molecular, liquid fuel I Theability of ajet engine to operate over a wide range of conditions isalso enhanced by the use of low molecular weight, high flame speedfuels. As the temperature and pressure of the inlet air tothe combustorare decreased, e.g., as by' an increase in altitude, it becomes moredifiicult to maintain stable and efiicient combustion. The use of'themore stably burning, gaseous, high flame speed fuels permits stable andeflicient combustion at lower inlet air temperature and pressure, and

thus increases the altitude operational range of the engine.

Since these low molecular weight fuels are gases under normal conditionsof temperature and pressure, they have not been used as jet aircraftfuelso'wing to the 2 obvious difiiculties fostered by their gaseousstate. ,Thus, a. method and means of increasing the combustion stabilityand efficiency ofthe usual liquid jet engine fuels are greatly desired.Such a method and means would not only increase the operational range ofthe engine and permit the decrease of its weight and frontal area, butwould also increase the availability (and concomitantly decrease thecost) of jet fuels by permitting the inclusion of fuel components oflower volatility, lower combustion stability, and lower combustionefficiency. Also, the performance increase provided by such a method andmeans can be taken as an increase in thrust per unit of engine Weightand unit of engine frontal area.

Thus, an improved combustion chamber for a continuous combustion burnerby which a portion of the fuel supplied to the combustion system isdecomposed into fuel components having high flame speed characteristicsis highly desirable and an object of this invention is to provide such acombustion chamber.

A further object is to provide a combustion chamber for a continuouscombustion burner wherein stable combustion occurs over a wide rangeofopera'ting conditions.

A further object is to provide a combustion chamber for a jet enginewherein problems of cycling and flame blowout under severe operatingconditions are reduced.

In accordance with this invention, an improved combustion chamber isprovided which permits a portion of the fuel supplied to the engine tobe stoichiometrically burned in air and the resulting combustionproducts are used to decompose at least a portion of the remainder ofthe fuel into fuel components having'high flame speed characteristics.The decomposition process ordinarily involves predominantly crackingreactions although other reactions, such as dehydrogenation reactions,which influence the formation of hydrogen and unsaturated components,also take'place. The products from decomposing a portion of the fuel bydirect heat exchange with the products of stoichiometric combustion arefuel component having high flame speeds such as hydrogen, acetylene,ethylene, propylene and the like. Broadly stated, the inventioncomprises a continuous combustion burner wherein a liquid'fuel is burnedin air in a combustion chamber and whereinfthe hot gases therebyproduced are exhausted from .the combustion chamber, means forconducting stoichiometric combustion of a portion of the fuel in air,means for quenching the stoichiometric combustion with the remainder ofthe fuel so as to partially decompose the remainder of the fuel in theabsence of oxygen. The products from the stoichiometric combustionandthe decomposition of the fuel are burned in the combustion chamber andthe hot gases thereby produced are exhausted therefrom to produce thrustor to perform other work. i

In a preferred embodiment of this invention, a small proportion of thetotal fuel for the jet engine is utilized as the source of'heat requiredin a pilot chamber to furnish the energy required for the decompositionreaction of the larger, remaining proportion of the fuel. In otherwords, the combustion system of this invention comprises quenchingcombustion products in one part of the combustion chamber with fuel,instead of air, and burning the fuel so decomposed in the quenching stepin another combustion Zone in which secondary air is used to quench thecombustion products.

The amount of fuel supplied to the pilot chamber for stoichiometriccombustion can vary over a wide range. The amount of fuel used in thepilot chamber usually amounts to at least one percent by weight of thetotal greater than 15 percent by weight of the total fuel;

V and spaced from the fuel decomposition chamber.

The combustion which is carried on in the pilot chamber isstoichiometric combustion so that the decomposition of the remainder ofthe fuel by direct heat exchange with the product of the stoichiometriccombustion is carried on in an atmosphere which is substantially free ofoxygen. The combustion in the pilot chamber is on a high level, and hightemperatures, usually higher than 4000" F., are developed therein.

Although, the combustion chamber Which'is the subject of this inventionis most advantageously used in a jet engine, the apparatus can beemployed in any continuous combustion-type power plant includingturboprop, turbojet and ram jet engines for aircraft and stationary gasturbines for generating power. 7

A better understanding of the combustion chamber of thisinvention willbe had by referring to the accompanying drawings, in which: 7 a

Figure 1 is an over-all view of a jet engine having incorporated thereina view, partially in section, of the improved combustion chamber of thisinvention; and,

Fig. 2 is a partial, sectional view of a modification of the combustionchamber shown in Figure 1. 7

Referring now to Figure l, a jet engine is shown having to a combustionchamber 7 by a compressor 9 which is V driven by a turbine 11 by meansnot shown.

Combustion chamber 7 is a streamlined chamber having an air inlet 13 andan exhaust gas outlet 15. A flame tube 17 is axially-positioned withinand spaced from combustion chamber 7. Flame tube 17 comprises aplurality of apertures or perforations 19 for the admission of primaryair thereto and a plurality of such perforations 21 for the admission ofsecondary air thereto. The downstream end of flame tube 17 is open,attached to'and coincident with common exhaust gas outlet 15 ofcombustion chamber 7. V

A fuel decomposition chamber 25 is axially-positioned within and spacedfrom the upstream portion of flame tube 17. Decomposition chamber 25 isopen at its upstream end adjacent to and spaced from the upstream end offlame tube 17 and is closed at its downstream end. The downstream end ofdecomposition chamber 25 is positioned generally intermediate to theupstream and downstream ends of flame tube 17. A plurality of fuelnozzles 27 are positioned in the fuel decomposition cham 'berdownstreamend. Nozzles 27 are positioned so as to direct the'flow of fuel towardthe upstream end of fuel decomposition chamber 25.

A pilot flame chamber 29 is axially-positioned within The upstream endof pilot flame chamber 29 is attached to the upstream end of flame tube17 so that the latter also serves as the upstream end for pilot flamechamber 29. At least one perforation 31 is located in the downstream endof pilot flame chamber 29. In the embodiment shown in Figure 1, anaxially-positioned perforation 31 is located in the downstream end ofchamber 29. A plurality of perforations 33 are positioned in theupstream end of pilot flame chamber 29. A fuel nozzle 35 isaxially-positioned in the upstream end of pilot flame chamber 29 and ispositioned so as to direct the flow of fuel toward the downstream end ofpilot flame chamber'29.

Liquid jet engine fuel from a fuel supply 37 is connected by a conduit39, a conduit. 43 and a conduit41 to fuel nozzles '27, and by conduit39, conduit 43, and a conduit 45 to fuel nozzle 35. A valve 47 ispositioned in conduit 43 to permit the amount of fuel passing to nozzle35 to be burned in pilot flame chamber 29 to be controlled so thatstoichiometric combustion takes place in pilot flame chamber 29.

In the operation of the jet engine incorporating the combustion chamberof my invention, a portion of the liquid fuel, preferably a minorproportion, is passed via conduits 39, 43 and 45 to fuel nozzle 35 andburned in a portion of the total air supplied from air inlet 13 and inall the air supplied through perforations 33 to pilot flame chamber 29under stoichiometric conditions. The remainder of the fuel is passed viaconduits 39 and 41 to fuel nozzles 27. The hot combustion products fromthe stoichiometric combustion in pilot flame chamber 29 are exhaustedtherefrom through perforation 31 and quenched by liquid fuel indecomposition chamber 25. Thus, the major portion of the liquid fuel isdecomposed by direct heat exchange with the products of stoichiometriccombustion in pilot flame chamber 29. The resulting combustion productsand decomposed fuel are exhausted from decomposition chamber 25 throughits open upstream end and burned in primary air in the upstream portionof flame tube 17. The resulting products from. combustion in theupstream portion of flame tube 17 are quenched with secondary airadmitted through perforations 21 in the downstream portion of flame tube17. The resulting hot combustion gases and heated air are then exhaustedto the atmosphere through outlet 15, turbine 11 and exhaust gas outlet5.

It should be noted that the embodiment shown in Figure 1 includes a pairof perforations 33 in the upstream end of pilot flame chamber 29.- Aplurality of such perforations, that is more than 2, can'be used. Thenumber and cross-sectional area of the perforations 33 will determinethe amount of air admitted to the pilot flame chamber and, sincestoichiometric combustion is conducted in the pilot flame chamber, will.determine the amount of fuel which is stoichiometrically burned. As waspointed out before, the amount of fuel stoichiometrically burned varieswith the characteristics of the fuel being used. In order to permitvariation of the cross-sectional area of perforations 33 so that thecombustion chamber can be adapted to jet engine fuels of varyingchemical and physical characteristics, a modification of the apparatusof my invention is shown in Figure 2 wherein the area of perforations 33can be varied. Thus it will be seen that the upstream end of pilot flamechamber 29 has four perforations 33 and has a pair of perforations. 31in the downstream end of chamber 29. A plate 49 is slidably attached tothe exterior of the upstream end of pilot flame chamber 29. Plate 49 hasa series of perforations Sland a large, axially-positioned, perforation53. Perforations 51 are of the same size or cross-sectional area asperforations 33. Plate 49 is slidable upon the exterior of the upstreamend of flame tube 29 by a mechanical linkage 55. Thus, by adjusting theposition of plate 49, it'is possible to reduce the total cross-sectionalarea of perforations 33 which is exposed to inlet air down to a. pointat which these perforations are completely closed. Obviously, thespecific arrangement for varying the area of perforations 33 is onlyschematically shown and other specific systems will be readily apparentto those skilled in the art. For example, such systems as butterflyvalves or iris diaphragms controllable froma single source can bemounted upon the perforations 33 to permit their crosssectional area tobe adjusted.

The decomposition reactions to which reference has been made ordinarilyinvolve predominantly cracking and depolymerization reactions althoughother reactions which influence the formation of hydrogen and lowmolecular Weight unsaturated hydrocarbons also take place. 'The reactionproducts formed in the decomposition zone contain a substantial amountof high flame speed gaseous components, such as hydrogen,acetylene,propylene and the like, admixed with a portion which isnormally liquid at atmospheric temperature and pressure. The improvedcombustion performance obtained by using the fuel mixtures formed by themethod and means of this invention *is believed to be the result ofsuperior piloting action of the high flame speed components in thecombustion chamber at the flame holding areas as well as the result ofpreheating, prevaporization and the occurrence of some pre-flamereactions.

The present invention providm a method of operating jet propulsion typeengines with a single fuel whereby higher combustion efficiency, higherheat release rates and higher combustion stability are obtained than waspreviously possible in conventional jet propulsion engines burning thesame fuel. This invention is particularly effective in producingimproved combustion stability and combustion efficiency in combustionsystems operated under rich mixture conditions. The present inventionalso permits certain fuels, which heretofore have not been considered tobe completely suitable fuels for jet propulsion engines because of lowvolatilities and excessive carbon deposition, to be effectively utilizedin jet propulsion engines.

The fact that jet fuels of relatively low volatility are rendered usefulby this invention is of importance because the newest high altitude,high Mach number aircraft and missiles place severe requirements ontheir fuels because of aerodynamic heating and decreased ambientpressures. These effects dictate the use of fuels of low volatility ifexcessive vapor losses from conventional vented fuel tanks are to beavoided. Also, such aircraft are limited in their performance by thevolume of fuel which they can carry and, therefore, the highervolumetric heating value of heavier petroleum fractions is a desirablefeature. However, this invention is applicable to combustion processesin general, such as stationary gas turbine power plants, and is notrestricted to aircraft jet engines such as turbojet, turboprop, and ramjet engines.

The following examples, calculations and data are supplied to illustratesome of the advantages afforded by the improved combustion chamber ofthis invention. The amounts of air and fuel required in the pilot flamechamber were calculated for a combustion system using a normal decanefuel and were based upon a total air requirement of 1.0 pound per secondand an over-all fuel-air ratio of .01. The energy required to decomposethe normal decane fuel at the rate of 0.01 pound per second according tothe following reaction was found to be 18.5 B.t.u. per second.

wherein C H is assumed to be 1,3-butadiene and C H is assumed to bel-butene. With a temperature drop of 3600 F. across the fueldecomposition chamber and using an average specific heat for air of 0.24B.t.u. per pound per degree F., the heat available from combustion ofthis amount of the fuel is 865 B.t.u. per poound of air. Since 18.5B.t.u. per second is the energy required to decompose the total amountof fuel, the amounts of fuel and air required in the pilot flame chamberis 0.0013 pound per second and 0.0200 pound per second, respectively.The stoichiometric fuel-air ratio for normal decan used was 0.066 andthe heat lost from the walls of the combustion chamber and the heatconducted from the combustion in the upstream end of the flame tube inthe primary air were neglected.

Another of the important advantages for the combustion chamber of myinvention is that it requires less space to produce an equivalent amountof heat than is required by a conventional combustion chamber. Aconventional combustion chamber operating on 20 pounds of air and 0.2pound of fuel per second to deliver 13,680,000 B.t.u. per hour requires6.4 cubic feet of space for the combustor. However, a combustorconstructed in accordance with this invention and operating under thesame conditions to yield the same amount of heat requires 6.0 cubic feetwhich is a saving of 0.4 cubic feet of space. The effect of temperatureor flame velocity has not been included in these calculations. However,it is believed that flame speeds would be at least two times those usedin these calculations at the operating temperature of the combustor.Therefore, the saving in space requirement would be even greater thanthat indicated by the foregoing calculations.

A further advantage for the combustion chamber of this invention is thatturbulent flow of air to the primary combustion zone is not required asis true in a conventional combustion chamber. In the latter, aconsiderable portion of the primary air must be turbulent in order toaid the vaporization of the fuel and this results in a pressure dropacross the combustor. For example when there is a 4 percent pressuredrop and a 8:1 pressure ratio in a conventional combustion chamber, eachpercent pressure drop results in a 1 percent drop in the efficiency ofthe power plant. However, in the combustion chamber of this invention,the fuel is vaporized completely before it leaves the fuel combustionchamber and therefore, the pressure drop for aiding the vaporization ofthe fuel is not required.

It will be obvious to those skilled in the art that many substitutions,changes and modifications can be made in the light of the foregoingdisclosure which will be Within the spirit and scope of my invention.

I claim:

1. A continuous combustion burner for burning a liquid fuel in air,which comprises, in combination, a combustion chamber having an airinlet at its upstream end and a product gas outlet at its downstreamend, a flame tube axially-positioned within and spaced from saidcombustion chamber, said flame tube having a plurality of perforationsabout the upstream portion thereof adjacent to said combustion chamberupstream end for admission of primary combustion air thereto and havinga plurality of perforations about the downstream portion thereofadjacent to said combustion chamber downstream end for admission ofsecondary combustion air thereto and having a product gas outlet at itsdownstream end attached to and coincident with said combustion chamberproduct gas outlet, an axially-positioned fuel decomposition chamberwithin, and spaced from said upstream portion of said flame tube, saidfuel decomposition chamber being open at its upstream end adjacent toand spaced from said flame tube upstream end and being closed at itsdownstream end, said decomposition chamber downstream end beingpositioned generally intermediate to said upstream and downstream endsof said flame tube, a plurality of first fuel nozzles positioned in saidfuel decomposition chamber downstream end, said first fuel nozzles beingpositioned so as to direct fuel toward said fuel decomposition upstreamend, an axially-positioned pilot chamber positioned within and spacedfrom said fuel decomposition chamber, the upstream end of said pilotchamber being attached to the upstream end of said flame tube so thatsaid flame tube upstream end also serves as the upstream end for saidpilot chamber, at least one perforation in said pilot chamber downstreamend, a second fuel nozzle axially-positioned in said flame tube upstreamend and positioned so as to direct fuel into said pilot chamber, aplurality of perforations in said pilot chamber upstream end, a firstconduit means for supplying fuel to said first fuel nozzles, a secondconduit connected to said first conduit means for supplying fuel to saidsecond fuel nozzle and a valve means positioned in said second conduitfor controlling the amount of fuel supplied to said second conduit sothat stoichiometric combustion of said fuel in air occurs in said pilotchamber.

2. A burner in accordance with claim 1 further comprising a means forvarying the cross-sectional area of said plurality of perforations insaid pilot chamber upstream end.

3. In a continuous combustion burner wherein a liquid fuel is burned inair in a combustion chamber comprising means for quenching thecombustion with air and means for exhausting the hot gases therebyproduced from said combustion chamber, a pilot flame chamber, a fueldecomposition chamber, said pilot flame chamber being positioned Withinthe fuel decomposition chamber and the fuel decomposition chamber beingpositioned Within the combustion chamber, means for stoichiometricallyburning a portion of said fuel in a portion of said air in one end ofthe pilot flame chamber, means for exhausting the products of combustionfrom the opposite end of the pilot flame chamber into the fueldecomposition chamber, means for introducing the remainder of said fuelinto the fuel decomposition chamber at one end thereof and adjacent tothe last mentioned exhaust means, and means at the opposite end of thefuel decomposition chamber for exhausting a mixture of combustion gasesand decomposed fuel gases into the combunstion chamber whereby saidgases pass in indirect heat exchange and countercurrent to thecombustion gases flowing through the pilot flame chamber.

4. The apparatus of claim 3 in which means is provided for introducingthe remainder of the fuel entering the fuel decomposition chamber incountercurrent contact with the products of combustion from the pilotflame chamber.

5. A continuous combustion burner for a fuel in air which comprises incombination a combustion chamber having an air inlet and a product gasoutlet, aflame tube positioned within the combustion chamber, adecomposition chamber within the flame tube, a pilot flame chamberwithin the decomposition chamber, means for stoichiometrically burning aportion of said fuel in a portion of said air in the pilot flamechamber, means for exhausting the products of combustion from the pilotflame chamber into the fuel decomposition chamber, means for introducingthe remainder of said fuel into the fuel decomposition chamber at oneend thereof and adjacent to the above mentioned exhaust means, means atthe opposite end of the fuel decomposition chamber for exhausting amixture of combustion gases and decomposition gases into the flame tubewhereby said gases are passed in indirect heat exchange andcountercurrent to the combustion gases flowing through the pilot flamechamber, means for passing primary air from the air inlet into the flametube about the decomposition chamber and means for passing secondary airfrom the air inlet into the flame tube down stream of the decompositionchamber to quench the combustion reaction therein.

6. A method of burning liquid fuel to power a jet engine which comprisesburning a first portion of said fuel in a pilot zone with astoichiometric amount of air to form hot combustion gases, contactingsaid hot gases in a decomposition zone with a second portion of saidfuel whereby the fuel thus contacted is decomposed in the absence of airto lower molecular weight fuels, passing the resulting mixture ofcombustion gases and said lower molecular weight fuels to a combustionzone, and burning said mixture with air to form combustion products forpowering said engine.

7. The process of claim 6 in which the amount of fuel stoichiometricallydecomposed comprises at least one percent of and not greater than 15.percent by weight of the total liquid fuel.

8. A method of burning liquid fuel to power a jet engine which comprisesburning a first portion of said fuel in a pilot zone with astoichiometric amount of air to form hot combustion gases, contactingsaid hot gases in a decomposition zone with a second portion of saidfuel whereby the fuel thus contacted is decomposed in the absence of airto lower molecular weight fuels, passing the resulting mixture ofcombustion gases and said lower molecular weight fuels to a combustionzone, said mixture en route passing in indirect heat exchange relationwith said pilot zone and said combustion zone, and burning said mixturewith air to form combustion products for powering said engine.

9. A method of burning liquid fuel to power a jet engine which comprisesburning a first portion of said fuel in a pilot zone with astoichiometric amount of air to form hot combustion gases, contactingsaid hot gases in a decomposition zone with a second portion of saidfuel whereby the fuel thus contacted is decomposed in the absence of airto lower molecular weight fuels, passing the resulting mixture ofcombustion gases and said lower molecular weight fuels to a combustionzone, said mixture en route passing in indirect heat exchange relationwith said pilot zone and said combustion zone, burning said mixture withprimary air to form combustion products for powering said engine, andquenching said combustion products with secondary air.

10. The apparatus of claim 5 in which a variable air inlet means isprovided for the pilot flame chamber.

11. The process of claim 9 in which the amount of feedstoichiometrically decomposed comprises at least one percent and notgreater than 15 percent by weight of the total liquid fuel.

References Cited in the file of this patent UNITED STATES PATENTS1,757,855 Chilowsky May 6, 1930 2,059,523 Hepburn et a1. Nov. 3, 19362,201,965 Cook May 21, 1940 2,206,553 Nagel July 2, 1940 2,523,096Clements Sept. 19, 1950 2,628,475 Heath Feb. 17, 1953 2,635,426 MeschinoApr. 21, 1953 2,655,786 Carr Oct. 20, 1953 2,697,910 Brzozowski Dec. 28,1954 2,767,233 Mullen et al. Oct. 16, 1956 FOREIGN PATENTS 712,843 GreatBritain Aug. 4, 1954

